1. Field of the Invention
The present invention relates to a semiconductor light emitting device having an end-facet window structure which prevents carrier recombination at a light-exit end facet. The present invention also relates to a process for producing such a semiconductor light emitting device.
2. Description of the Related Art
In conventional semiconductor light emitting devices, when optical output power is increased, currents generated by optical absorption in vicinities of end facets generate heat, i.e., raise the temperature at the end facets. In addition, the raised temperature reduces the semiconductor bandgaps at the end facets, and therefore the optical absorption is further enhanced. That is, a vicious cycle is formed, and the end facet is damaged. This damage is the so-called catastrophic optical mirror damage (COMD). Thus, the maximum optical output power is limited due to the COMD. In order to overcome the above problem, various techniques have been proposed for the window structures which prevent the light absorption in the vicinities of end facets by increasing the semiconductor bandgaps in the vicinities of the end facets.
For example, Japanese Unexamined Patent Publication No. 2000-31596 discloses a semiconductor laser device and a process for producing a semiconductor laser device. In the process, a window structure is realized by removing a portion of an upper cladding layer in a vicinity of light-exit end facet to a depth near a quantum-well active layer by etching, and forming a regrowth layer doped with the same dopant as that of an upper cladding layer so that the dopant diffuses into the quantum-well active layer, and crystal mixture occurs in the quantum-well active layer.
Since, in the above process, the dopant is diffused into the quantum-well active layer through the cladding layer and an optical waveguide layer during the formation of the regrowth layer, the diffusion depth of the dopant and the degree of the crystal mixture vary due to irregularity of thermal diffusion occurring during the formation of the regrowth layer, and therefore the window structure has poor reproducibility. Thus, it is difficult to produce, at a high yield rate, the above semiconductor laser device so that the semiconductor laser device is reliable in a high output power operation. In addition, in the above process, three semiconductor-layer growing steps and two dry etching steps are required to be performed until a semiconductor laser chip is completed. That is, the manufacturing process is complicated, and the manufacturing cost is high.
An object of the present invention is to provide a semiconductor light emitting device which is reliable in a high output power operation.
Another object of the present invention is to provide a process which can produce a semiconductor light emitting device being reliable in a high output power operation, through a small number of manufacturing steps with high reproducibility.
(1) According to the first aspect of the present invention, there is provided a semiconductor light emitting device comprising: a layered structure and a regrowth layer formed over the layered structure. The layered structure includes a lower cladding layer of a first conductive type, an active zone including an active layer, an upper cladding layer of a second conductive type, and a current confinement layer, which are formed on a substrate in this order. In the layered structure, a groove having a depth corresponding to a bottom of said current confinement layer or a lower elevation is formed for realizing a current injection window, and at least one space is formed between at least one near-edge region of the active zone and at least one end facet. The regrowth layer is doped with a dopant which makes the regrowth layer the second conductive type, and formed over the layered structure so that the groove and the at least one space in the layered structure are filled with the regrowth layer, and the dopant is diffused into at least one near-edge region of the active layer.
The first conductive type is different in the polarity of carriers from the second conductive type. That is, when the first conductive type is an n type, and the second conductive type is a p type.
In the semiconductor light emitting device according to the first aspect of the present invention, at least one space is produced between at least one end facet and the at least one near-edge region of the active zone, and is filled with the regrowth layer so that the dopant with which the regrowth layer is doped is diffused into the at least one near-edge region of the active layer. Therefore, crystal mixture occurs in the at least one near-edge region of the active layer, and the energy gap in the at least one near-edge region of the active layer is increased. Thus, the light absorption in the at least one near-edge region of the active layer is reduced. In particular, even when the output power is increased, and the temperature rises, the light absorption in the at least one near-edge region of the active layer can be reduced. Therefore, it is possible to obtain a semiconductor light emitting device which is reliable even in a high output power operation.
Preferably, the semiconductor light emitting device according to the first aspect of the present invention may also have one or any possible combination of the following additional features (i) to (iv).
(i) A first partial thickness of the layered structure corresponding to the depth of the groove has such composition that the first partial thickness of the layered structure can be etched off by wet etching concurrently with the active layer and a second partial thickness of the layered structure above the active layer.
When the semiconductor light emitting device according to the first aspect of the present invention has the above additional feature (i), and the at least one space between the at least one end facet and the at least one near-edge region of the active layer is produced by forming a lamination of the lower cladding layer, the active zone, the upper cladding layer, and the current confinement layer on the substrate, and etching off at least one near-edge portion of the lamination having a depth corresponding to the bottom of the active layer or a lower elevation, and the groove is produced by etching off a stripe portion of the lamination being located inside the at least one space and having the depth corresponding to the bottom of the current confinement layer or a lower elevation, the etching-off operations for producing the at least one space and the groove can be performed concurrently by an identical lithography process or a self alignment process. Therefore, the number of manufacturing steps can be reduced.
(ii) The substrate is made of GaAs, the lower cladding layer and the upper cladding layer are made of InGaP or AlGaAs, the active layer is made of InxGa1xe2x88x92xAs1xe2x88x92yPy (0xe2x89xa6xxe2x89xa60.4, 0xe2x89xa6yxe2x89xa60.1), and the current confinement layer is made of InGaP of the first conductive type. The active zone further includes a lower optical waveguide layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 (x3=0.49y3, 0xe2x89xa6x3xe2x89xa60.3) of an intrinsic type or the first conductive type and formed under the active layer, and an upper optical waveguide layer made of Inx3Ga1xe2x88x92x3As1xe2x88x92y3Py3 (x3=0.49y3, 0xe2x89xa6x3xe2x89xa60.3) of an intrinsic type or the second conductive type and formed above the active layer. The layered structure further includes a buffer layer made of GaAs and formed between the substrate and the lower cladding layer, and an etching stop layer made of GaAs and formed under the current confinement layer. The lower optical waveguide layer may be arranged immediately under the active layer, and the upper optical waveguide layer may be arranged immediately above the active layer.
In addition, the InGaP material is a semiconductor material containing at least indium, gallium, and phosphor as components, and the AlGaAs material is a semiconductor material containing at least aluminum, gallium, and arsenic as components. Further, preferably, the compositions of the lower and upper cladding layers are such that the lower and upper cladding layers lattice-match with the GaAs substrate. In particular, it is preferable that the composition of the InGaP material is In0.49Ga0.51P, and the composition of the AlGaAs material is Al0.5Ga0.5As.
(iii) The at least one space has an approximately identical width with a width of the groove, and is located adjacent to the groove.
(iv) The second conductive type is a p type, to and the dopant is Zn. In this case, the bandgap in the vicinity of the end facet can be satisfactorily increased. Therefore, the light absorption in the vicinity of the end facet can be reduced, and high reliability can be maintained even in a high output power operation.
(2) According to the second aspect of the present invention, there is provided a process for producing a semiconductor light emitting device, comprising the steps of: (a) forming a substrate; (b) forming above the substrate a lower cladding layer of a first conductive type; (c) forming above the lower cladding layer an active zone including an active layer; (d) forming above the active zone an upper cladding layer of a second conductive type; (e) forming a current confinement layer above the upper cladding layer, and obtaining a lamination of the lower cladding layer, the active layer, the upper cladding layer, and the current confinement layer; (f) removing at least one first near-edge portion of the lamination being located near at least one of two opposite end facets and having a first depth and a width corresponding to a stripe width so as to produce at least one first space, by etching the lamination with a first mask which has an opening corresponding to the first near-edge portion; (g) removing at least one second near-edge portion of the lamination being located near the at least one of the two opposite end facets and having the stripe width and a second depth corresponding to the bottom of the active layer or a lower elevation, and a stripe portion of the lamination being located inside the at least one first space and having the stripe width and a third depth corresponding to the bottom of the current confinement layer or a lower elevation, so as to produce a second space, by etching the lamination with a second mask which has a stripe opening having a width corresponding to the stripe width and extending from one to the other of the two opposite end facets; and (h) forming a regrowth layer so as to fill the at least one first space and the second space.
The above first width corresponds to a width of a current injection region.
According to the second aspect of the present invention, in the step (f), a first partial thickness of the near-edge portion of the lamination (i.e., the at least one first near-edge portion of the lamination) having the width corresponding to the stripe width is removed by etching. Thereafter, in the step (g), a second partial thickness of the near-edge portion of the lamination (i.e., the at least one second near-edge portion of the lamination) and having the stripe width is removed by using a mask which has an opening having the stripe width so that at least the full thickness of a cross section of the active layer is exposed, and at the same time, a stripe portion of the lamination being located inside the at least one first space (produced in step (f)) and having the stripe width is removed by using the same mask so that at least the full thickness of a cross section of the current confinement layer is exposed. Thus, the formation of the stripe groove for the internal stripe structure and the removal of the near-edge portion of the active zone can be concurrently achieved by using the same mask. In other words, the conventional manufacturing process of forming the end-facet window structure and the conventional manufacturing process of forming the internal stripe structure can be substituted by a common step of lithography and etching and a common step of forming a regrowth layer, although each of the above conventional manufacturing processes includes a step of forming a step of lithography and etching and a step of forming a regrowth layer. Thus, the total manufacturing process can be simplified by the second aspect of the present invention.
In addition, since the at least one near-edge space for the end-facet window structure and the groove for the internal stripe structure are formed by one lithography process, the at least one near-edge space produced by the removal of the at least one near-edge portion is well aligned with the stripe groove without any provision for realizing the alignment, and the reproducibility in the formation of the end-facet window structure and the groove for the internal stripe structure is high.
Further, since the near-edge portion of the active zone is removed, and is thereafter filled with the regrowth layer, the dopant of the second conductive type in the regrowth layer can be diffused into the at least one near-edge portion of the active layer with high reliability and reproducibility during the formation of the regrowth layer.